1. Field of the Invention
The present invention relates to an imaging lens and an image reading apparatus using it and, more particularly, to those capable of reading an image at high accuracy by properly correcting curvature of field, astigmatism, etc. of the imaging lens in reading of image information in the line-sequential reading method by use of an image pickup element such as CCD or the like and suitably applicable, for example, to such apparatus as image scanners, copiers, facsimile machines, and so on.
2. Related Background Art
FIG. 1 is a schematic diagram of a major part of an example wherein the conventional imaging lens for reading of an image is used in a flat-bed image scanner.
In the same figure light emitted from an illumination light source 701 illuminates an original 708 directly or via a reflector 709, an optical path of reflected light from the original 708 is bent by first, second, third, and fourth reflecting mirrors 703a, 703b, 703c, 703d inside a carriage 706, and the reflected light is focused on a surface of linear image sensor 705 (hereinafter represented by xe2x80x9cCCDxe2x80x9d) such as the CCD (Charge Coupled Device) or the like by the imaging lens 704. The carriage 706 is moved in a direction of an arrow A (sub scanning direction), illustrated in FIG. 1, by a sub scanning motor 707 to read the image information of the original 708. The CCD 705 in the same figure is composed of a plurality of light receiving elements arrayed in a one-dimensional direction (main scanning direction).
In the above structure it is necessary to reduce the size of the carriage 706 in order to decrease the size of the image scanner. For downsizing of the carriage 706, there are methods of maintaining the optical path length, for example, by increasing the number of reflecting mirrors or by reflecting the light several times by a single reflecting mirror.
These methods, however, greatly increase the cost, because the internal structure of the carriage 706 becomes complicated, so as to require strict assembling accuracy. They also degrade the imaging performance in proportion to the surface precision of the reflecting mirrors and the number of reflections, so as to affect the read image as well.
Another conceivable method is to decrease the object-to-image distance by increasing the angle of view of the imaging lens (imaging system) 704 to a wider angle. Various types of imaging lenses have been suggested with a wide angle of view that could be realized in a practical number of lenses and in a spherical surface shape. However, they have an upper limit of about 25xc2x0 to a half angle of view and thus setting an angle of view wider than this increases the curvature of field and astigmatism, so as to result in a failure in demonstrating adequate optical performance.
FIG. 2 is a lens sectional view of Numerical Example A, detailed hereinafter, of a conventional imaging lens and FIG. 3 is a diagram to show various aberrations of Numerical Example A, described hereinafter, of the imaging lens illustrated in FIG. 2. The imaging lens in FIG. 2 is constructed in the structure of a telephoto type having five lenses, which are a positive, first lens 91, a negative, second lens 92, a stop, a positive, third lens 93, a negative, fourth lens 94, and a negative, fifth lens 95 arranged in the named order from the object (original) side. The imaging lens in the same figure is set toward a goal of use at the half view angle of 30xc2x0, but astigmatism becomes larger at view angles greater than approximately 70% in the object height (near the half view angle of 22xc2x0), as shown in the aberration diagram of FIG. 3. It is hard to correct the astigmatism further while suppressing the other aberrations.
There were many suggestions to use imaging lenses having much wider view angles by introducing a general, aspherical surface of a rotationally symmetric shape to the above type or the like to correct wavefront aberration, but it was not easy to realize a sufficiently wide view angle, because no fundamental solution was given to the curvature of field, astigmatism, and so on.
A method for correcting the astigmatism is, for example, the image reading apparatus suggested in Japanese Patent Application Laid-Open No. 5-14602. In the application, astigmatism is corrected well by placing an optical member having refracting power rotationally asymmetric in the normal directions to the optical axis, in an optical path between the imaging system and the image reading means. This method is effective to correct astigmatism, but necessitates a new optical member placed in the optical path, thus posing issues of an increase in the scale of the overall apparatus and an increase in adjustment items in assembly.
The apparatus can have high resolving power at all the view angles by making an eclipse at wide view angles, but the image reading apparatus is required to have an aperture efficiency of 100%. Therefore, this approach also fails to accomplish the above object.
An object of the present invention is to provide an imaging lens capable of achieving sufficient imaging performance with a small number of lenses even at ultra-wide angles while correcting for curvature of field, astigmatism, etc. well, by providing at least one surface out of a plurality of surfaces forming the imaging lens with a refracting power rotationally asymmetric with respect to the optical axis, and also providing an image reading apparatus using it.
An imaging lens of the present invention is an imaging lens for use in image reading, for imaging image information of an original on reading means, wherein at least one surface out of a plurality of surfaces forming the imaging lens has a refracting power rotationally asymmetric with respect to the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface of a rotationally asymmetric shape and is integral with a surface of a single lens forming the imaging lens.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a diffracting optical element having a rotationally asymmetric refracting power and is integral with a surface of a single lens forming the imaging lens.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is formed by adding a member of a rotationally asymmetric shape with respect to the optical axis to a rotationally symmetric surface.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is formed by adding a diffracting optical element having a refracting power rotationally asymmetric with respect to the optical axis, to a rotationally symmetric surface.
An image reading apparatus of the present invention is aranged to image the image information of the original illuminated by light from an illumination light source on a surface of the reading means and read the image information, using the imaging lens described above.
Another imaging lens of the present invention is an imaging lens for use in image reading, for imaging image information of an original on reading means, the imaging lens having a stop in the imaging lens, wherein at least one surface not facing the stop, out of a plurality of surfaces forming the imaging lens, has a refracting power rotationally asymmetric with respect to the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface of a rotationally asymmetric shape having a rotationally symmetric refracting power on the optical axis and is integral with a surface of a single lens forming the imaging lens.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface the radius of curvature of a generating line of which is equal to the radius of curvature of a meridian line thereof on the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is formed by adding a member of a rotationally asymmetric shape with respect to the optical axis to a rotationally symmetric surface.
Another image reading apparatus of the present invention is arranged to image the image information of the original illuminated by light from an illumination light source on a surface of the reading means and read the image information, using the imaging lens described above.
Another imaging lens of the present invention is an imaging lens for use in image reading, for imaging image information of an original on reading means, wherein at least one surface at which an overlapping area of a range of up to 70% of a distance from a center to the outermost periphery of each of on-axis rays and outermost off-axis rays is not more than 50% of an area of a range of up to 70% of the distance from the center to the outermost periphery of the on-axis rays, out of a plurality of surfaces forming the imaging lens, has refracting power rotationally asymmetric with respect to the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface of a rotationally asymmetric shape having a rotationally symmetric refracting power on the optical axis and is integral with a surface of a single lens forming the imaging lens.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface the radius of curvature of a generating line of which is equal to the radius of curvature of a meridian line thereof on the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is formed by adding a member of a rotationally asymmetric shape with respect to the optical axis to a rotationally symmetric surface.
Another image reading apparatus of the present invention is arranged to image the image information of the original illuminated by light from an illumination light source on a surface of the reading means and read the image information, using the imaging lens described above.
Another imaging lens of the present invention is an imaging lens for use in image reading, for imaging image information of an original on reading means, the imaging lens comprising five lenses, which are a positive, first lens of the meniscus shape with a convex surface thereof facing the side of the original, a second lens whose lens surfaces both are concave surfaces, a third lens whose lens surfaces both are convex surfaces, a positive or negative, fourth lens of a meniscus shape with a convex surface thereof facing the side of the reading means, and a negative, fifth lens of the meniscus shape with a convex surface thereof facing the side of the reading means, arranged in the named order from the original side,
wherein at least one surface out of a plurality of surfaces forming the imaging lens has a refracting power rotationally asymmetric with respect to the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface of a rotationally asymmetric shape having a rotationally symmetric refracting power on the optical axis and is integral with a surface of a single lens forming the imaging lens.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is the surface a radius of curvature of a generating line of which is equal to the radius of curvature of a meridian line thereof on the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is formed by adding a member of a rotationally asymmetric shape with respect to the optical axis to a rotationally symmetric surface.
The present invention is also characterized by further comprising a stop between the second lens and the third lens.
Another image reading apparatus of the present invention is arranged to image the image information of the original illuminated by light from an illumination light source on a surface of the reading means and read the image information, using the imaging lens described above.
Another imaging lens of the present invention is an imaging lens for use in image reading, for imaging image information of an original on reading means, the imaging lens comprising four lenses, which are a positive, first lens of the meniscus shape with a convex surface thereof facing the side of the original, a second lens whose lens surfaces both are concave surfaces, a third lens whose lens surfaces both are convex surfaces, and a negative, fourth lens of the meniscus shape with a convex surface thereof facing the side of the reading means, arranged in the named order from the original side,
wherein at least one surface out of a plurality of surfaces forming the imaging lens has a refracting power rotationally asymmetric with respect to the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface of a rotationally asymmetric shape having a rotationally symmetric refracting power on the optical axis and is integral with a surface of a single lens forming the imaging lens.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface the radius of curvature of a generating line of which is equal to the radius of curvature of a meridian line thereof on the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is formed by adding a member of a rotationally asymmetric shape with respect to the optical axis to a rotationally symmetric surface.
The present invention is also characterized by further comprising a stop between the second lens and the third lens.
Another image reading apparatus of the present invention is arranged to image the image information of the original illuminated by light from an illumination light source on a surface of the reading means and read the image information, using the imaging lens described above.
Another imaging lens of the present invention is an imaging lens for use in image reading, for imaging image information of an original on reading means, wherein at least one surface out of a plurality of surfaces forming the imaging lens has a refracting power rotationally asymmetric with respect to the optical axis,
wherein in such a coordinate system that an origin lies at an intersecting point of the one surface with the optical axis, x along a direction of the optical axis, y along a reading direction of the reading means, and z along a direction perpendicular to the optical-axis direction x and to the reading direction y,
letting R(y) be the radius of local curvature at a certain point in the reading direction y on an xy section of the surface, and r(y) be the radius of local curvature in a direction perpendicular to the xy plane and normal to the shape of the xy section in the reading direction y,
the following condition is satisfied in a range of the reading direction y from 0 to a passing position yxe2x80x2 of an outermost off-axis principal ray;
0xe2x89xa6|[R(y)xe2x88x92r(y)]/[fd(Ndxe2x88x921)]|xe2x89xa60.15
where
fd: focal length of the overall system of the imaging lens at the d-line; and
Nd: index of refraction of a lens with said one surface formed therein, at the d-line.
The present invention is also characterized in that the R(y) satisfies the following condition in the range of the reading direction y from 0 to the passing position yxe2x80x2 of the outermost off-axis principal ray:
0xe2x89xa0|dR(y)/dy|.
The present invention is also characterized in that the R(y) and the r(y) are equal to each other on the optical axis.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is a surface of a rotationally asymmetric shape and is integral with a surface of a single lens forming the imaging lens.
The present invention is also characterized in that the surface having the rotationally asymmetric refracting power is formed by adding a member of a rotationally asymmetric shape with respect to the optical axis to a rotationally symmetric surface.
The present invention is also characterized in that the reading means is a linear image sensor.
Another image reading apparatus of the present invention is arranged to image the image information of the original illuminated by light from an illumination light source on a surface of the reading means and read the image information, using the imaging lens described above.